Tab Electronics Guide to Understanding Electricity ... - Sciences Club

234 Chapter Eight
Figure 8-2
A hypothetical class
AB audio amplifier.
V
R1
RC
Q3
C1
+
P1
Q2
RE3
+
C2
RE4
R2
Q1
Q4
RE
voltage as the forward biased base-emitter junction of each output transistor.
The problem with this method is a lack of adjustment. If the forward
threshold voltage of each diode is not exactly equal to the
base-emitter junction voltage of each transistor, some crossover distortion
can occur. If three diodes are used, the quiescent conduction current
of each output transistor might be too high, resulting in excessive
heating of the output transistors.
The amplified diode circuit could also be replaced with an adjustable
biasing resistor for biasing purposes. Although this system will function
well and eliminate crossover distortion, the adjustable resistor will not
thermally “track” with the output transistors. As you might recall, bipolar
transistors have a negative temperature coefficient, meaning that they
exhibit a decrease in resistance with an increase in temperature. In reference
to transistors, a decrease in resistance actually means an increase in
leakage current. In other words, bipolar transistors become more “leaky”
when they get hot. In bipolar transistor amplifiers, this is a major problem.
As output transistors begin to heat up, the leakage current also
increases, causing an increase in heat, causing an increase in leakage current,
causing an additional increase in heat, causing an additional
increase in leakage current, and so forth. This condition will continue to
degrade until the output transistors break down. A breakdown of this
nature is called thermal runaway.
A means of automatic thermal compensation is needed to correct the
problem. An adjustable resistor cannot do this (most resistors have a positive
temperature coefficient), but that is the beauty of an amplifier diode